INTRODUCTION. 



In striking contrast witli what has occurred in other groups of lower 

 Vertebrates, the increase of known species of Chelonians has been 

 very slight within the last twenty years. In fact, the present 

 volume records a considerably smaller number of species than did the 

 last-published treatise, viz. Dr. Gray's " Supplement to the Catalogue 

 of Shield lieptiles," in 1870. This decrease is of course due to the 

 different views held by the writers as to what deserve to constitute 

 Bpecific characters, and also, I trust, to a better understanding, at 

 the present day, of the amount of variation within given formf--. 

 Uut the fact that hardly 20 valid species have been discovered sin< e 

 the close of Dr. Gray's labours shows that our knowledge of the 

 existing Chelonians is far nearer completion than that of any other 

 group of Reptiles, Crocodiles excepted. 



Though deficient in novelties, — not a single new species or genus 

 is established, — I trust that the present account will prove of interest 

 for the thorough revision which higher and lower groups have 

 undergone, and for the information it contains on the osteologicul 

 characters, which have hitherto been much neglected. Figures of 

 the skulls and shells (stripped of the epidermal shields, and show- 

 ing the arrangement of the latter as well as of the underlying bony 

 plates*) are given for the principal genera. Although imperfect 

 as regards the sutures and other details, the woodcuts of the skulls 

 given by Dr. Gray in some of his papers are still useful to show 

 the shape and general characters of the skulls, and therefore have 

 been reproduced in the present volume. As to the original figures, 

 which have been executed with great care, I hope they will prove 

 welcome to palaeontologists as well as to zoologists. 



* Crosses have been inserted in these figures to indicate the position of the 

 axillary and inguinal buttresses of the plastron, which anchj lose with the lower 

 surface of the carapace. 



